Agglomeration of Ti-SBA-15 with clays for liquid phase olefin epoxidation
in a continuous fixed bed reactor.
Juan A. Melero,*,† Jose Iglesias,† Javier Sainz-Pardo,† Pilar de Frutos,‡ Sandra Blázquez‡
† Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, C/Tulipán s/n, E-28933 Móstoles, Madrid, Spain
‡ Repsol-YPF Research Center, E-28931 Móstoles, Madrid, Spain.
Published on:
Chemical Engineering Journal 139 (2008) 631-641
doi:10.1016/j.cej.2008.01.013
* To whom correspondence should be addressed:
Abstract
Titanium-containing SBA-15 material has been agglomerated with bentonite clay to form macroscopic structured catalyst particles with the purpose of being used in continuous epoxidation processes on a fixed bed reactor. The binding conditions, the mass ratio catalyst to binding agent as well as the calcination temperature of the material were studied to improve as much as possible the catalytic behavior and mechanical strength of the titanium-based catalyst. The binded material has been finally tested in the liquid epoxidation of 1-octene with ethyl benzyl hydroperoxide in a continuous up-flow fixed bed reactor. Reaction results reveal a better catalytic performance of this catalyst, both regarding to the conversion of the olefin and efficient use of the oxidant, than conventional commercially available TiO2-SiO2, specially when pellet surface hydrophobization is performed by silylation treatment.
Keywords
1. Introduction
type materials. Incorporation of titanium species into the matrix of SBA-15 type mesostructured materials can be achieved by post-synthesis grafting treatments, involving the reaction between the silica support and a solution containing the titanium starting material (16),(17), and direct synthesis procedures (18). Latter strategy of synthesis usually leads to more stable and dispersed supported metal species than other methods, however its use involves more difficulties mainly caused by the strong acidic conditions for the synthesis of SBA-15 materials. Recently, we have reported that some of the troubles related to the very low pH value of the synthesis media for the incorporation of the metal species can be partially overcome by a proper selection of the titanium precursor (18). Titanocene dichloride as titanium source and proper synthesis conditions allow to obtain Ti-SBA-15 samples showing a good metal dispersion, free from anatase domains and displaying a high accessibility to the titanium sites (18). These materials have shown a very high catalytic activity in the epoxidation of olefins with alkyl hydroperoxides under batch conditions.
2. Experimental
2.1 Materials and methods
Titanocene dichloride [Cl2TiCp2, 97%] was used as titanium source for the synthesis of titanium functionalized mesostructured materials and purchased from ABCR. Non-ionic surfactant [Pluronic 123, EO20PO70EO20, M=5800], tetraethylorthosilicate [TEOS, 98%], hexamethyldisilazane [HMDS, 99%] and 1-octene [99%] were purchased from Aldrich Chemicals. Ethylbenzyl hydroperoxide [EBHP, 50% wt in ethylbenzene] was kindly provided by Repsol-YPF to be used as oxidant in epoxidation reactions. The oxidant has been employed as received without previous purification. Commercial TiO2-SiO2 catalyst (Shell type catalyst) was also provided by Repsol-YPF as 2 mm spherical pellets as catalyst reference (21). This reference material has been widely studied in the epoxidation of different olefins using several oxidation agents, from the simplest hydrogen peroxide to the bulky ethylbenzyl hydroperoxide, showing for all of them good performance in epoxidation reactions and efficient use of the oxidant. Chemical analysis of the material revealed titanium content of 0.96% in weight basis.
2.2 Synthesis of powder Ti-SBA-15
Surfactant was removed by calcination in air at 550ºC for 5 hours under static conditions. The solid was then recovered as a white fine powder.
2.3 Macroscopically structured Ti-SBA-15 materials
Agglomeration of powder Ti-SBA-15 catalysts with clays has been performed using different procedures. All of the methods make use of inorganic agglomerant clay (bentonite or sepiolite) and an organic additive (methyl cellulose, 5% wt.). Mixing all of the components with ultra pure water leads to a homogeneous mixture which is kneaded to achieve an uniform distribution of the liquid. The processing of the resultant composition varies depending on the agglomeration method:
Method A consists on drying the mixture on air at 110ºC during 24 hours followed by a crushing step and a final sieving to achieve a material with particle size comprised between 0.7 and 1.0 mm. The resultant particles were then calcined in air under static conditions using a heating ramp of 0.3ºC·min-1 up to 550ºC during 2 hours.
Method B comprises an extrusion step of the raw mixture to form rods. The drying step is carried out in a drying chamber equipped with a humidity controller under the following program: First the extruded rods were subjected to 20ºC and relative humidity of 70% during 24 hours. Then, the humidity was reduced down to 35% keeping constant the temperature conditions for 24 hours. Final stage comprises 80ºC and relative humidity of 10% during 20 hours. The dried rods were then calcined in air at 550ºC for 2 hours using a heating ramp of 0.5ºC min-1. The final materials were then crushed and sieved to get particles sized between 0.7 and 1.0 mm.
length particles. The rest of the stages make use of the same conditions to those described for method B although in this case the crushing step is avoided.
Figure 1 outlines the main steps and final materials obtained through each one of the agglomeration methods.
2.4 Silylation of agglomerated materials
The binded Ti-SBA-15/bentonite catalyst was treated with HMDS using a similar procedure to that previously reported in literature (18). In a typical preparation, a suitable amount of freshly agglomerated material was outgassed at 150ºC during 24 hours under vacuum. The material was then reacted for 24 hours with a dry mixture of toluene/HMDS in a Soxhlet apparatus under N2 atmosphere (mass ratio material/HMDS of 1.0). The treatment was carried out under static conditions in order to avoid particle attrition. The silylated material was then washed several times with toluene a dried under nitrogen atmosphere for 4 h at 200ºC.
2.5 Catalyst characterization
analysis. N2 adsorption-desorption isotherms were recorded at 77K in a Micromeritics Tristar manometric porosimeter. The specific surface areas were calculated using the B.E.T. method whereas the pore size distributions were determined by the application of the B.J.H. method to the adsorption branch of the isotherm using a Harkins-Jura equation for the adsorbed multilayer thickness specially obtained for SBA-15 type materials (22). Thermogravimetric analyses were carried out on a SDT simultaneous 2960 microscale analyzer from TA Instruments. The different contributions to the weight lost have been distinguished through the deconvolution of the DTA signal using Lorentz curves and subsequent integration. Diffuse reflectance ultraviolet spectra (DR-UV-Vis) were collected in a VARIAN CARY-500 spectrophotometer equipped with an integration sphere accessory in the wavelength range from 200 to 600 nm. FTIR analysis were performed using a Mattson infinity series spectrophotometer using the KBr buffer technique and recording the spectra in the wavenumber range from 4000 to 400 cm-1. XRF analyses were performed on a Philips MagiX spectrometer. The mechanical resistance of agglomerated materials was measured by two different methods, using a standard assay for determining the bulk crushing strength (BCS) (23) and measuring the individual particle resistance. The latter one was carried out as follows: a single particle is subjected to increasing pressure until particle breaks. This procedure is repeated ten times to obtain the mean value.
2.6 Catalytic epoxidation of 1-octene with ethyl benzyl hydroperoxide
heated up to 110ºC. Once the temperature is reached, the oxidant is added in a unique step by means of a syringe (EBHP: 1-octene molar ratio of 0.12) and all the conditions are then kept constant for two hours. The epoxidation reactions performed under continuous flow were carried out in a glass-made fixed bed reactor like that shown in Figure 2. The catalytic fixed bed, 1.2 cm of inner diameter and 15 cm length (catalyst loading of 3.0 g), is located in a jacketed glassware tube between two beds of spherical glass inert particles. The temperature of the system is controlled (110 / 120ºC) on top of the bed by using an external heating bath recycling a stream of silicone oil through the jacket of the reactor. Typically, the feed solution mixture, containing both 1-octene and oxidant, was loaded into the fixed bed reactor in up-flow operation by means of a Gilson 10SC HPLC pump operating at 1.0 mL·min-1 (calculated spatial time as the weight of catalyst to mass feed flow was of 3.6 min). EBHP composition was varied in order to determine the influence of the molar ratio oxidant to substrate in the catalytic behaviour of agglomerated Ti-SBA-15 materials. Under the reaction conditions, epoxide and methyl benzyl alcohol (MBA) as oxidant by-product were the reaction products. In both cases, either using the batch reactor or the fixed bed, the hydroperoxide conversion was calculated by iodometric titration whereas the rest of the reaction products were quantified by gas chromatography analysis.
3. Results and discussion
investigated. Finally the macroscopically casted catalyst has been tested in the epoxidation of 1-octene in liquid phase in a continuous up-flow fixed bed reactor.
3.1 Thermal treatment of binding agent
The choice of the binding agent has been carried out attending to its activity in the decomposition rate of the oxidant after different thermal treatments. Clays are known to show a high concentration of hydroxyl groups responsible for the Brönsted acidity of these materials (24)-(26). These acid centers are the responsible for the non-oxidative consumption of alkyl hydroperoxides (27)-(29). Thus, the selection of the proper binding agent includes also an additional study on the removal of their hydroxyl groups.
Figure 3 shows the TGA analysis carried out for bentonite and sepiolite materials. The detected weight losses have been correlated with different types of hydroxyl groups removed during calcination. Thus, in the case of the bentonite clay(Figure 3A), the mass change detected in the first and second regions, which comprise up to 300ºC, is attributed to the removal of adsorbed water and water molecules located at narrow pores strongly adsorbed onto the surface of the clay. The third region, comprising the range from 300ºC to 550ºC includes the removal of the water molecules located between the bentonite plates. Finally, weight loss detected above 550ºC is assigned to the dehydroxylation of the material by condensation of silanol groups evolving water molecules and leading to a collapse of the clay structure.
micropores of the clay structure and finally the rest of the weight loss is attributed to the removal of the solvating water molecules around magnesium ions. Thus, in the temperature range used for this study, no dehydroxilation processes corresponding to the removal of silanol groups has been detected, which is in accordance with previously reported results (30). Higher temperature values were not assayed because of damage of the porous structure of the mesoporous material might be caused in the binded Ti-SBA-15/clay materials (10).
modified. On the other hand, rehydration of magnesium ions seems not to be easy once the water molecules have been removed from its coordination sphere, since the contribution of this region (region III) to the total weight loss is not significative after rehydration by contact with atmosphere.
Having in mind, as previously noticed, the quantity of silanol and hydroxyl functionalities located at the surface of the clays is an indirect measurement of their Brönsted acidity, materials calcined at such a high temperature to cause the dehydroxilation of the material by condensation of silanol groups should lead to a lower decomposition extent of alkyl hydroperoxides. In this sense, if the assumed conclusion is right, only the bentonite samples calcined at temperatures above 550ºC, should lead to poor hydroperoxide conversion. Figure 4 shows the results achieved in the EBHP decomposition tests carried out in presence bentonite and sepiolite samples treated at different temperatures. These tests were developed in presence of a 7% wt. solution of ethylbenzyl hydroperoxide in ethylbenzene for 2 hours at 80ºC.
samples calcined at 550ºC and 650ºC show different content of surface hydroxyl groups, they promote almost the same hydroperoxide consumption.
On the other hand, sepiolite samples cause higher decomposition degrees of the hydroperoxide than those achieved with bentonite materials, whatever the calcination temperature used in the preparation of the samples. Since the elemental analysis by XRF do not provide meaningful differences related to the presence of transition metals able to catalyze the decomposition of hydroperoxides, differences between sepiolite and bentonite in the decomposition of hydroperoxide have been ascribed to the differences in the population of surface hydroxyl groups. Unlike bentonite, sepiolite samples have not been highly dehydroxylated even at such a high temperature as 650ºC. In this sense, all the sepiolite materials tested in this study show some fraction of their original hydroxyl groups content, and thus the clay is still able to catalyze the degradation of EBHP. Therefore, it can be concluded that there is a direct correlation between the decomposition degree of EBHP and the concentration of hydroxyl groups located onto the surface of the clays used in this study. Anyway, increasing the calcination temperature causes a gradual lowering of hydroperoxide conversion, being this effect more accentuated for bentonite materials than for sepiolite samples. Since calcination at 550ºC is enough to achieve the minimum hydroperoxide conversion with bentonite, this clay, as well as 550ºC have been selected as binding agent and calcination temperature respectively for the preparation of the agglomerated Ti-SBA-15 materials described in the rest of this research.
3.2 Composition of the agglomeration raw mixture
of the final particles and the concentration of catalytic sites in the final material. Thus, a high proportion of binding agent provides high mechanical resistance but the catalytic behaviour of the final material may be greatly reduced because of an excessively low amount of active catalyst. Thus, this study pursues compromising the mass ratio of binding agent to achieve agglomerated materials with high catalytic activity and mechanical resistance. The amount of binding agent has been varied between 10% and 40% wt. in the agglomeration raw mixture. The materials were prepared by the agglomeration method A (see experimental section) to achieve a particle size between 0.75 mm and 1.0 mm.
Figure 6A depicts the trend of the titanium loading as well as the particle resistance versus the amount of bentonite in the final materials. The use of increasing amounts of binding agent causes a decreasing of the final metal content in the materials because of an expected dilution effect. In contrast, the higher loading of bentonite the more resistant agglomerated particles. In this sense, it seems to be an inflection point of the trend in the particle resistance around a mass composition in bentonite of 25% - 30% wt. Higher clay loadings do not provide increasing particle resistance but, on the contrary, causes an unnecessary dilution of the active catalyst reducing in this way the proportion of titanium species. This dilution can exert negative influence on the catalytic behaviour of the agglomerated materials in catalytic epoxidation of olefins. Thus, in order to evaluate properly which is the true influence of the binding between bentonite and Ti-SBA-15 materials, some catalytic assays have been performed using agglomerated materials as catalysts in a stirred batch reactor in presence of EBHP as oxygen source. Figure 6B shows the correlation between the bentonite loading and the titanium content with the conversion of the substrate. It is noticeable that all the materials displayed similar values for the efficiency in the use of the oxidant towards the epoxide (~
In this context, the optimal amount of bentonite for the preparation of agglomerated Ti-SBA-15 materials seems to be ca. 30 wt %. because it leads to an important increasing in the particle resistance whereas the amount of titanium present in the resulting material is not largely reduced. Larger amounts of bentonite do not provide increasing particle mechanical resistance whereas it reduces the activity of the final material because of dilution of the Ti-SBA-15 catalyst.
3.3 Enhancement of mechanical resistance and hydrophobicity of catalyst particles.
obtained through the different agglomeration methods do not show significant differences regarding to their textural and structural parameters. In the same sense, all the agglomeration procedures lead to materials showing almost the same catalytic behavior, both regarding to the conversion of the substrate and the efficiency in the use of the hydroperoxide, indicating the modifications introduced on each agglomeration method do not cause differences on the catalytic activity of the final materials. However, differences regarding to the particle resistance are quite noteworthy since controlling the drying step as well as using an extrusion procedure leads particles with increasing mechanical resistance, probably as a consequence of the lower stress caused by the mild conditions used during the controlled drying procedure. The slow removal of water from extruded rods avoids the formation of microscopic cracks in the solid, jeopardizing the particle resistance. Additionally, it is noticeable the significant increasing achieved in the mechanical strength when casting the particles by cutting before drying and calcination (method C) instead of crushing calcined rods (method B). This better behavior is assigned to the lower mechanical stress suffered by particles when applying this method which leads to materials as resistant as the commercial catalysts used as reference material (SiO2/TiO2), as it is inferred from almost the same BCS index achieved for both materials (Table 3).
way the efficiency in the use of the oxidant (18). For this reason, post-synthesis silylation has been performed on the material prepared through the agglomeration method C, that showing the higher particle resistance, as well as the commercial catalyst based on SiO2-TiO2. The physico-chemical properties, as well as their behavior in catalytic tests have also been included in Table 3. Although some textural properties, like the surface area and pore volume are partially modified by the silylation process, because of the grafting of trimethyl silyl functionalities onto the inner porosity, the particle resistance is not altered at all, probably as a consequence of the gentle conditions employed for the silylation treatment of agglomerated particles. On the contrary, the efficiency in the use of the oxidant is enhanced when using silylated materials as catalysts. In this case this parameter reaches an acceptable value of ca. 85 % for the silylated Ti-SBA-15 / bentonite material.
3.4 Liquid phase epoxidation of 1-octene with EBHP in an up-flow fixed bed reactor
12% mole of oxidant in the feedstock solution, the catalytic activity decreases compared to catalytic assays with a less concentrated oxidant. This fact seems to be related with the observed trend for the efficiency in the use of the oxidant, which is dramatically reduced from 100% to 60% when increasing the concentration of EBHP. These results suggest that the secondary reactions involving hydroperoxide, like the non oxidative consumption, are enhanced when increasing the concentration of this reagent in the reactants stream, becoming as important as the epoxidation reaction.
explained because of the different composition of both solids. Thus, the commercial catalyst is based on 99% wt. silica and thus, the amount of hydroxyl functionalities is much higher than that located at the agglomerated material before silylation. Hydrophobization treatment is not so effective in the removal of hydroxyl groups for the commercial catalyst than for the mesostructured material and thus, the behaviour of both treated materials regarding to the efficient use of the hydroperoxide becomes different.
4. Conclusions
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Table 1. Weight loss contributions in TG analysis performed on bentonite and sepiolite clays calcined at different temperatures. Values in wt %.
Bentonite Sepiolite
T (ºC) Reg I Reg II Reg III Reg IV T (ºC) Reg I Reg II Reg III
r.t. 1.51 1.53 2.13 1.67 r.t. 1.43 3.69 3.07
300 0.69 1.40 2.07 1.69 300 1.11 2.53 2.82
400 0.60 1.17 2.08 1.69 400 0.99 1.20 2.32
550 0.40 1.07 0.10 1.53 550 1.02 1.93 0.64
Table 2. Physicochemical properties of Ti-SBA-15 / bentonite materials
Sample Bentonite
a
(%)
SB.E.T. b
(m2·g-1)
Vp c (cm3·g-1)
Dp d (Å)
Ti-SBA-15 0 809 0.95 92
S-1 10 620 0.81 85
S-2 20 546 0.72 85
S-3 30 447 0.64 88
S-4 40 403 0.59 90
Table 3. Physicochemical properties and catalytic activity of materials agglomerated by different methods
Synthesis treatments Physico-chemical properties Catalytic activity a Agglomeration
method
Post-synthesis treatment
Ti b (%)
SB.E.T. c
(m2·g-1)
Vp d (cm3·g-1)
Dp e (Å)
Rp f (MPa)
XOCT g
(%)
X’OCT h
(%)
XEBHP i
(%) EBHP
j
(%)
Method A --- 0.57 407 0.64 88 2.52 3.45 63.9 91.1 70.1
Method B --- 0.52 407 0.62 87 3.13 3.52 65.1 90.4 72.0
Method C --- 0.59 403 0.62 86 5.94 (0.74) 3.42 63.3 91.5 69.2
Commercial
TiO2-SiO2 --- 0.96 334 0.87 160 5.90 (0.72) 3.44 63.7 85.6 74.4
Method C Silylation 0.53 365 0.55 80 5.94 3.24 60.0 70.4 85.2
Commercial
TiO2-SiO2 Silylation 0.86 294 0.85 154 5.90 3.41 63.1 78.7 80.2
Table 4. Steady state results for the epoxidation of 1-octene in a fixed bed reactor under continuous flow.
Run Catalyst EBHP (%) a X (%) OCTb X’(%) OCT c X(%) EBHP d EBHP e
(%)
A f (mmol·gcat-1·h-1)
ATig
(mmol·gTi-1·h-1)
1
Ti-SBA-15 / Bentonite Silylated
1.5 1.5 56.5 57.8 97.8 1.02 192
2 3.0 3.0 54.3 62.0 87.6 1.97 372
3 6.0 3.2 28.6 33.0 86.8 2.08 392
4 12.0 2.5 11.7 19.0 61.5 1.69 319
5 Commercial TiO2-SiO2 silylated 3.0 2.5 47.3 96.4 49.1 1.71 199
Figure Captions
Figure 1. Agglomeration methods for the preparation of macroscopically structured
Ti-SBA-15 materials
Figure 2. Scheme of the up-flow fixed bed reactor for the epoxidation of 1-octene in liquid
phase under continuous flow.
Figure 3. Thermogravimetric analysis and weight losses assignation for A) bentonite and
B) sepiolite clays.
Figure 4. EBHP decomposition over bentonite and sepiolite clays after different thermal
treatments.
Figure 5. Nitrogen adsorption-desorption isotherms recorded at 77K for agglomerated
Ti-SBA-15 materials with different loadings of bentonite. A) Recorded scale and B)
Normalized scale.
Figure 6. Correlation between the bentonite loading and A) particle mechanical resistance
and titanium loading and B) catalytic activity in the liquid phase epoxidation of 1-octene
for different clay loadings. (1) conversion of 1-octene referred to the maximum
Ti-SBA-15 Methyl Cellulose Binding Agent
Bentonite
Sepiolite Plasticity
Water Kneadding
Wet mixture Drying
Sieving
Calcination
Extrusion
Cutting Drying & Calcination
Method A Method B Met hod C
Crushing
Drying & Calcination Sieving
Extruded rod
Wet Mixtur e
L
=
2
m
m
D=2 mm
Figure 1
FT FIC CATALYST FIXED BED
REACTION MIXTURE (EB, EBHP,
1-Octene)
TT FIXED BED REACTOR FOR LIQUID
PHASE OLEFIN EPOXIDATION
PRODUCT STREAM (EB, EBHP,
1-Octene, epoxide,
MBA)
Figure 2
100 200 300 400 500 600 700 91 92 93 94 95 96 97 98 99 100
100 200 300 400 500 600 700 91 92 93 94 95 96 97 98 99 100 Weig ht (%) A Structure collapse Reg IV Interlaminar water Reg II Adsorbed water molecules Reg III Reg I Temperature (ºC) B Physisorbed water Magnesium ions dehydration Reg II Adsorbed water molecules in micropores Reg III Reg I Temperature (ºC) Figure 3
300 400 500 600 700 0
10 20 30 40 50 60 70 80 90 100
2 h
1 h
1 h 2 h
E
B
HP
deco
mpo
si
ti
o
n
(%
)
Calcination Temperature (ºC)
Bentonite Sepiolite
Figure 4
0.0 0.2 0.4 0.6 0.8 1.0 0 100 200 300 400 500
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 In cr ea si n g b en to n it e lo adi n g A Ads orbed volu m e (cm
3 ST
P
·g
-1 )
Partial pressure (P/P 0) B Re lat ive ads orbed volu m e (Un it les s)
Partial Pressure (P/P 0)
Figure 5
10 20 30 40 0.4 0.5 0.6 0.7 0.8 40 45 50 55 60 65 70 R elat ive oc tene co nve rsio n(% ) (1 ) Bentoni te (%)
Tit anium
Conte nt(%
)
10 15 20 25 30 35 40
0.45 0.50 0.55 0.60 0.65 0.70 0.75 1.6 1.8 2.0 2.2 2.4 2.6 2.8 M as s ti ta n iu m co n te n t (% ) % Bentonite Optimal composition P ar ti c le re si st an ce (M P a) A B Figure 6
